Watermelons are natural diploids, referred to as 2N (N=11). Many plants, including watermelons, can undergo a duplication of their entire set of chromosomes and exist as tetraploids 4N (4N=44). Watermelon tetraploids can be produced routinely in the laboratory using cell biology techniques.
A tetraploid parent can be crossed with a diploid parent to produce triploid seeds (3N=33). A hybrid triploid plant produces watermelon fruit which is “seedless,” meaning that it very rarely produces mature seeds, and only rarely produces immature seeds with hard seed coats, but no embryo. To obtain triploid watermelon seed, a cross between a tetraploid and diploid line is made. The tetraploid female flower is used as the ‘seed parent’ and the diploid male flower is used as the pollen donor. Seeds obtained from this cross bear triploid embryos. A tetraploid seed parent typically produces only 5 to 10% as many seeds as a typical diploid plant. If female diploid flowers are pollinated with pollen coming from tetraploid flowers the result is empty seeds.
When production of triploid hybrids is done by open pollination, there is no control on the source of pollen that reaches the stigma of the tetraploid flower, and thus both tetraploid and triploid seeds may be found in the same fruit. In the absence of selection methods, hybrid seeds produced in this manner may therefore be contaminated with seeds resulting from self-pollination.